1. Technical Field
The present invention relates generally to fuel control systems and, more particularly, to a method of determining the percent alcohol content of a fuel used in a motor vehicle capable of operating on more than one type of fuel.
2. Discussion
Environmental and energy independence concerns have stimulated the development of alternative transportation fuels, such as alcohol fuels, for use in automobiles. Alcohol fuels include methanol and ethanol. A flexible fueled vehicle capable of operating on gasoline, or alcohol fuel, or any mixture of the two fuels, is therefore in demand. Modifications to the engine are necessary when operating on different fuels because of the different characteristics of each fuel. For example, an engine operating on ethanol or E85 (a blend of 85% ethanol and 15% gasoline) requires approximately 1.4 times the amount of fuel relative to gasoline at stoichiometry due to a lower energy content of the ethanol.
Air/fuel ratio in internal combustion engine design is typically considered to be the ratio of mass flow rate of air to mass flow rate of fuel inducted by an internal combustion engine to achieve conversion of the fuel into completely oxidized products. The chemically correct ratio corresponding to complete oxidation of the products is called stoichiometric. If the air/fuel ratio is less than stoichiometric, an engine is said to be operating rich, i.e., too much fuel is being burned in proportion to the amount of air to achieve perfect combustion. Likewise, if the air/fuel ratio is greater than stoichiometric, an engine is said to be operating lean, i.e., too much air is being burned in proportion to the amount of fuel to achieve perfect combustion. Alcohol fuels have a lower air/fuel ratio than gasoline at stoichiometric, so that the engine must be compensated for in the rich direction as the percentage of alcohol in the fuel increases.
U.S. Pat. No. 5,255,661, entitled "Method for Determining Fuel Composition Using Oxygen Sensor Feedback Control", hereby expressly incorporated by reference, discloses a method for determining the percent alcohol content of fuel in the fuel tank utilizing an oxygen sensor feedback control loop to sense changes in air/fuel ratio and relay that information to the engine controller so that dependant variables can be adjusted accordingly. However, oxygen sensors take a predetermined amount of time to warm-up to a reliable state, particularly on cold starts. As such, fuel composition learning systems cannot be used until an oxygen sensor is functioning correctly after the vehicle has been filled with fuel.
A potential for drivability deficiencies exist during the warm-up time frame if the vehicle has been recently filled with a fuel blend (i.e., alcohol and gasoline) which differs from the old fuel blend in the fuel tank. Drivability deficiencies often occur if the blended fuel after the fill has entered the intake manifold and new fueling parameters have not yet been determined. Therefore, it would be desirable to provide a method for determining the percent alcohol content of a newly blended fuel prior to its delivery to the intake manifold and prior to oxygen sensor feedback availability.